The invention relates to the general field of fuel injectors in turbomachines, and more particularly it relates to cooling main injectors in a two-headed combustion chamber of such a turbomachine.
A turbojet or a turboprop (referred to below in the present description as a xe2x80x9cturbomachinexe2x80x9d) having a twoheaded combustion chamber is started and kept idling using so-called xe2x80x9cpilotxe2x80x9d injectors only, while xe2x80x9cmainxe2x80x9d injectors are additionally brought into use while cruising. Pilot injectors are fed with fuel on a permanent basis, whereas main injectors are fed only once the turbomachine is rotating at more than some minimum determined speed (generally lying in the range 10% to 30% of its nominal speed). Furthermore, during so-called xe2x80x9cstage burningxe2x80x9d, only half of the main injectors are in operation, with the other half of the main injectors then being temporarily stopped.
Unfortunately, while idling, and even more during stage burning, it is necessary to cool the main injectors, and most particularly to cool their ends that extend into the combustion chamber (often referred to as xe2x80x9ctipsxe2x80x9d) in order to avoid coking problems.
Various injector architectures have been proposed to resolve this problem. Thus, in its French patent application No. FR 2 721 694, the Applicant has disclosed a main injector which is locally cooled by the fuel feeding a pilot injector, which fuel is conveyed via a central duct to the end of the injector and is returned via a coaxial annular duct. American patent No. U.S. Pat. No. 6,003,781 discloses a main injector provided with an independent cooling circuit, the cooling fluid being taken to the end of the injector via a top inlet channel and being returned via a bottom return channel.
Nevertheless, those prior art systems suffer from the same major drawback, that of cooling the end portion of the injector in localized manner only, thereby leaving entire areas that are not cooled. As a result, under certain operating conditions, particularly at high temperatures, e.g. around 900xc2x0 C., the end portion of the injector is not cooled sufficiently so it is not possible to avoid coke forming.
The present invention seeks to provide a cooling circuit that makes it possible in main injectors to avoid such formation of coke at high temperature. An object of the invention is thus to provide complete protection for the fuel circuits of such injectors. Another object of the invention is to provide such a circuit in a manner that is simple and without significantly altering the size of the injectors. Yet another object of the invention is to provide a cooling circuit that prevents maximum effectiveness in terms of extracting the from the fuel.
These objects are achieved by a system for cooling an injector of a combustion chamber of a turbomachine, said injector comprising means for delivering a primary fuel comprising a first feed tube connected to an annular injection piece having first injection orifices for discharging the primary fuel into said combustion chamber; and means for delivering a secondary fuel comprising a second feed tube surrounding said first feed tube and connected to a cylindrical endpiece surrounding said annular injection piece and having second injection orifices for discharging the secondary fuel into said combustion chamber, said endpiece further comprising an annular channel of diameter greater than that of said second feed tube and extending over its entire length beyond said first injection orifices; the system comprising means for delivering a cooling fluid comprising a third tube surrounding said second tube and having a tubular separation element connected thereto which is introduced in said annular channel of said cylindrical endpiece so as to form two annular spaces in which the cooling fluid can flow over 360xc2x0 all the way to the end of the injector.
With this particular structure, cooling is provided uniformly all the way to the far end of the tip of the main injector, where the temperature is the highest, and above all cooling is performed completely (i.e. over 360xc2x0) and not merely locally as in prior art systems.
Advantageously, the first and second feed tubes and the third tube are coaxial and the annular injection piece is connected to said first feed tube through a cylindrical connection piece.
The present invention also provides a fuel injector for a turbomachine combustion chamber, the injector including a cooling system as defined above.